This thesis considers infant social development from the viewpoint of the perceptual and memory capacities necessary for particular social abilities. Some social abilities, e.g. facial or voice discrimination, require visual or auditory integrity, thus the development of visual and auditory capacities are reviewed. Recognition of familiar faces and/or voices requires memory. Hence the development of memory abilities is considered. Subsequently the development of social behaviour is reviewed. After these literature reviews, three experimental studies are described. The first of these investigates the recognition of mother's voice and reports evidence of that such recognition develops during the first month of life. The second experiment considers visual recognition of the mother and differential responsivity to face-to-face and averted gaze and to different tones of voice. One month old infants did not reveal any conclusive evidence on these points. However, post-hoc analysis suggested the importance of the physical characteristics of faces in eliciting infant visual attention. Experience in these studies suggested the need for the study of more naturalistic encounters and hence a methodology for the study and analysis of naturalistic social interactions was developed. This methodology was then applied to a study of interactions between mothers and strangers with infants seen from one to eight months of age. This study revealed a surprising developmental pattern of differentiation between mother and stranger, with an unexpected period of positive responsiveness to strangers occurring at five months of age. The sequential analysis of interactions revealed evidence of a progressive development of infant receptivity to gaze, and also an exploratory analysis of receptivity to adult smiles and vocalizations suggested infants may respond to these adult behaviours. Subsequently the results of these studies are linked to other recent research.